The invention relates generally to rotary drill bits. More particularly, the invention relates to systems and methods for leaching Polycrystalline Diamond (“PCD”) cutter elements.
Oil and gas drilling operations often employ fixed cutter drill bits to drill through various rock formations in an effort to access hydrocarbon reserves below the ground. Fixed cutter drill bits employ a plurality of cutter elements that engage, scrape, and shear the earthen formation being drilled through. Such cutter elements are typically made of a layer or table of Polycrystalline Diamond (“PCD”) bonded to a cobalt cemented, tungsten carbide (WC) substrate.
To manufacture PCD tables for cutter elements and bond the tables to the substrate, diamond powder is placed at the bottom of a first mold or can along with a catalyst. The substrate is then placed on top of the diamond powder within the first mold, a second mold or can is placed on top of the substrate, and a seal is formed between the first and second cans. This entire assembly is then subjected to high pressure and temperature conditions to form a PCD cutter element. In general, any Group VIII element (e.g., cobalt, nickel, or iron) can be used as the catalyst, however, in most cases, cobalt (Co) is employed. The catalyst is driven into the interstitial spaces between the diamond grains and promotes intergrowth therein, to form a solid PCD diamond table suitable for use in a cutter element. The high pressure and temperature conditions also facilitate bonding between the newly formed PCD table and the substrate, thereby resulting in a fully formed PCD cutter element.
During drilling operations, cutter elements experience relatively high temperatures due, at least in part, to the general nature of the downhole environment and friction between the cutter elements and the formation. The thermal loads result in expansion of the material components of the cutter elements. Due to differences in the coefficients of thermal expansion between the catalyst and the diamond grains, at sufficiently high temperatures, undesirable cracks can form in the PCD lattice structure. These cracks can lead to failure of the corresponding cutter element(s), reduced cutting efficiency, and reduced cutting effectiveness. In addition, high thermal loads can lead to the undesirable formation of compounds such as, for example, carbon monoxide, carbon dioxide, or graphite within the PCD table itself. The presence of such compounds in the PCD table can further reducing the cutting effectiveness and strength of the corresponding PCD cutter element. Accordingly, it is generally desirable to remove at least a portion of the catalyst from the PCD table after its formation to enhance cutter element durability over a broader range of operating temperatures.
A common approach for removing the catalyst from a PCD table is to leach the PCD table to remove some or substantially all of the interstitial catalyst from the PCD lattice structure, thereby transforming the PCD material into thermally stable polycrystalline diamond. Leaching typically involves placing the cutter element in a strong acid bath at an elevated temperature to expose the PCD table to the acid. Suitable acids for leaching include nitric acid, sulfuric acid, hydrofluoric acid, hydrochloric acid, and combinations thereof. Although such leaching acids can aid in removing the catalyst from the PCD table, they can also damage the underlying substrate to which the PCD table is secured.
Conventional leaching via acid bath is a relatively time-consuming as it may take days or even weeks to remove a sufficient quantity of the binding agent from the PCD table. This increases the overall time, and associated costs, to manufacture cutter elements and fixed cutter drill bits.